75mm traverse

[Jerry Carducci]

I must confess that I've been spending my time on another project but I'm returning to my Lee and want to finish resolving the few outstanding items
with it. I'm going to finish the painting by following through on my interpretation of an early-issue American service M3 in North Africa. Color photos are virtually non existant so I'm going to use 'Modeler's license'

I successfully implemented the sbus to pwm converter/interface/decoder device which gave me enough channels to cover all the functions with several to spare.

The one item that resists my efforts to function normally is the traverse for the 75mm. The servo, servo mount and steel actuating (connecting?) arm to connect to the bottom of the 75mm mount are all OEM Armortek parts( part of the recoil pack) and well made robust parts. The 75mm 'drum' rotates fine by hand using the 75 barrel as a handle and using the breech end of the gun it still rotates well. The servo once connected to the bottom of the drum with the aforementioned steel arm simply strains and buzzes the lament of an unhappy servo and the 75 doesn't budge. I'm sure it's a question of leverage. I have longer 25T servo arms and I can make a longer actuating arm but I'm not sure that will solve the issue. When the servo attempts to rotate 75 mount the entire thing moves ever so slightly sideways and I believe this causes it to lock up as it's unsupported at the bottom. Up to now I've left the pattern of screws around the top of the mount loose as I thought these may have caused binding but perhaps the looseness is the issue.

My question is: how did you other builders solve this issue if you had the issue at all? I'd like to follow someone else's proven solution rather than make up a bunch of parts that may or may not solve the issue. In the meantime I'm going to try other solutions and if I find one that works reliably I'll post it here.

Jerry

[Stephen White]

Jerry, we’ve been working on this with Chris Hall’s M3. The solution we’ve gone for in principle is twofold:

- reduce friction and binding between the top flange of the barbette and the hull mounting. We scimmed the top surface of the barbette 0.5mm to remove machining marks from production and ensure clearance with the top cover plate.

- support the bottom of the barbette. We made a support bracket across the hull piece under the barbette, into which we drilled and tapped an M5 bolt modified with a conical end. We will centre drill the centre point of the barbette to receive it and act as an adjustable bearing.

We were conscious of the potential for a foul with the surrounding hull structure and indeed, when Chris got it home I gather it did foul. We’ll adjust it this week and test it. If all’s well we can share the solution.

Another approach might be to fit a lazy susan type bearing if one exists that small.

“Wait out, SiTREP to follow”

[Chris Hall]

It is an issue …..

The gun drum spins on the top lip, which introduces friction. Grease only partially helps. You also need to make sure the top plate isn’t in contact with the top of the drum.

But the main issue, as you say, is that the drum isn’t supported underneath, which means the servo has to work overtime (if at all). Stephen White and I (well, actually just him ) are working on a solution, which I’ll hopefully be able to post up later this week.

Good luck !

Chris

[Jerry Carducci]

Thanks for the replies chaps. I see that the the bottom of the barbette being unsupported is the source of my problems with this
set of parts. To the end of eliminating friction, in my case downright binding between barbette and top cover I approached it with two modifications.

First i mounted the cover in a lathe and machined off ,25-.38mm ( .010-015") from the underside. Second I made a paper
shim using normal copier paper that sits in the band where the circular pattern of screws is that fastens the cover to the casemate.
I have successfully eliminated all binding even after having all screws installed and properly tightened and lateral/vertical movement
of the barbette is minimal. I used a grease as found in our drive motors- a tacky red grease. It didn't help.

After trying this with the existing servo and by manually moving the steel arm connected to the bottom of the barbette I fond that the
servo is just barely able to move the barbette clockwise but completely unable to reverse the direction. I observed that the dynamics
are such that when the barbette is pushed from its bottom that it's able to be moved as long as it's in the direction where the lower portion is
supported by the front of the casemate but in the opposite direction the rear of the barbette being essentially unsupported it is able to cant ever
so slightly and bind to such a degree that the servo is incapable of overcoming the resistance. This a long winded way of saying my observations agree with yours. I'm sure there are some conditions under which everything is just perfect and the original implementation works as intended but mine isn't one of those and needs assistance.

Two solutions come to mind. Either the barbette needs to be rotated closer to the top where the flange is or as you both have said implement a support for the lower portion so it can't do anything but rotate so the servo has a fighting chance to move it.

During my tests the traverse (slew) servo became so hot from exertion that I feared I had lost it. I removed it and mounted it to a fan i use in my garage to cool it off! It was nearly too hot to handle. I've never seen a servo get that hot before; fortunately it still works ok.

The work continues...

Jerry

[Jerry Carducci]

Happily I was able to come up with a solution for this issue.



When approaching this I knew I didn't want to disassemble the entire 75mm assembly so one of my criteria was that I had to be able to modify the
existing mechanism in place. I suspect my solution will be unique to my model based on how I assembled it: to be able easily to remove the entire upper hull or to be able to remove just the fighting compartment's roof by itself leaving the rest of the upper hull alone- I wanted this to facilitate battery maintenance and access.

While the idea adding a bearing to the lower part of the barbette /casemate was very appealing to me with temperatures approaching 40c I had little
appetite for spending untold hours at my lathes and mill making parts. I thought seriously about removing the barbette, cutting a bearing groove on the bottom and making a corresponding piece to be attached to the casemate to make in effect a radial-lazy susan bearing built into the parts. But no not today. My goal was simple is good, the less time spent in the heat the better.

The photos I've attached should explain what I did. I only needed to make one new hole in existing hull parts and only a few new parts altogether.
Key to this solution was relocating the point on the barbette upon which to apply torque to a point closer to the supported end of it; I relocated this point to the top of the unit just below and within the radius of the supported rim. Care was taken that no newly added screws would cause interference with any moving surface nor does the addition of this modification interfere with existing functions. I was able to leverage the rivet holes along the edge
onf the top of the casemate as I didn't use these having had my roof removable; these rivet holes, 3 of them became 3mm threaded holes
with which I was able to mount my new servo fixture. As heavy as the barbette is it actually takes very little effort to move it this way; the original servo is more than up to the task and shows no signs of strain. I'm pleased with the way this came out. I can move on to finishing this model and final painting.

Jerry

[Vince Cutajar]

Nice one Jerry.

So if I am understanding correctly, the balljoint is now connected to the top of the casement instead of the bottom of the casement.

Vince

[Jerry Carducci]

Nice one Jerry.

So if I am understanding correctly, the balljoint is now connected to the top of the casement instead of the bottom of the casement.

Vince

In the interest of clarity I'm going to change my terminology. That component that holds the 75mm gun which turns to enable traverse
I'm going to refer to as the rotor - ala Richard Hunnicutt AND David Doyle's reference. It is to the top of this rotor that the ball joint is attached.
I've referred to the armored structure that houses the rotor as the casemate- To the side of the upper casemate flange is where the servo
is attached. The mention in David Doyle's book refers to the casemate as the rotor housing so there it is. Rotor housing it is.

It's all about nomenclature....

I forgot to mention previously that in my case this implementation serves a dual purpose. Not having the servo mounted to the lower hull
facilitates my removal of the entire upper hull without having to worry about forgetting the that servo's mechanical connection. I have
been known to do silly things like that.

Jerry

[Robert E Morey]

Not having a Lee I can't fully diagnose anything. But the gun rotation drum or rotor (for side to side movement) should be supported top and bottom - preferably by bearings on both ends (even plastic washer or bronze bush). Otherwise any amount of deflection in the rotation drum looks like it will cause binding and then overstress the servo. Moving the servo push rod to supported end should help, but not eliminate the issue. Sorry if my terminology of the parts isn't correct.

From your video it looks like you've solved the issue?
B

[Stephen White]

I didn't think I'd ever question the expertise of David Doyle, doyen of tank nomenclature, but the correct term is a barbette. The rotating thingy is properly called a barbette because it isn't a full turret. The term derives from medieval fortifications but in this context it comes from the use in naval design, where a barbette was used to provide protection to a gun with limited traverse, thus saving the weight of a full turret. It gave its name to a class of ship of which the British Admiral class of the late 19th Century was an example.

[Jerry Carducci]

Not having a Lee I can't fully diagnose anything. But the gun rotation drum or rotor (for side to side movement) should be supported top and bottom - preferably by bearings on both ends (even plastic washer or bronze bush). Otherwise any amount of deflection in the rotation drum looks like it will cause binding and then overstress the servo. Moving the servo push rod to supported end should help, but not eliminate the issue. Sorry if my terminology of the parts isn't correct.

From your video it looks like you've solved the issue?
B

Yes, yes I have.

Jerry

[Stephen White]

I didn't think I'd ever question the expertise of David Doyle, doyen of tank nomenclature, but the correct term is a barbette.

Jerry, must be an age thing, I was thinking of Hilary Doyle, whose authority is huge. I've not read any of the other Mr Doyle's books but nevertheless, barbette is appropriate. Rotor generally implies something that turns through 360 degrees, which the M3 barbette clearly doesn't. Who cares, we know what we mean? We should have something to report tomorrow on our evolving solution.

[Jerry Carducci]

I didn't think I'd ever question the expertise of David Doyle, doyen of tank nomenclature, but the correct term is a barbette.

Jerry, must be an age thing, I was thinking of Hilary Doyle, whose authority is huge. I've not read any of the other Mr Doyle's books but nevertheless, barbette is appropriate. Rotor generally implies something that turns through 360 degrees, which the M3 barbette clearly doesn't. Who cares, we know what we mean? We should have something to report tomorrow on our evolving solution.

No worries.

"The cylindrical thing that moves withe the 75mm", "barbette", "rotor", "drum", or the time honored Americanism- "doohickey" with the big gun on it
all work.

I was able to get it, whatever it is to work reliably to my satisfaction so there it is.

"You Say Tomáto, I say Tomäto..."


Jerry

[Chris Hall]

Other solutions may be available ...........

The key issues are (a) how to reduce friction, and (b) how to reduce 'wobble', given that the drum just 'hangs' within the sponson. I could imagine a solution, but it's way above my technical ability. So, fortunately, Stephen White offered to help, proving (yet again) my growing adage that "it's not what you know, it's who you know, what they know, and what clever bits of kit they have" . In this case, Stephen provided the technical knowledge, the milling machine, the lathe, and various taps and dies. So the credit for this alternative solution is down to him - my input was confined to a few helpful suggestions (only half of which were any use at all ) and hoovering up the swarf.

We both felt that the assembly needed to be supported from below, to reduce the friction of the small flange at the top. A suitable piece of aluminium bar was therefore cut, milled, filed, drilled and tapped (I at least know the words !) to produce this:

It needed tapping and rebating from both sides - the inside hole can be left, but the outside hole will need to be filled in. And then an M5 bolt (all the bolts are M5) was suitably drilled into the centre to provide a pivot point, lifting the whole assembly just a touch to reduce that flange friction. There was also a lot of material that needed to be milled out to fit over the top of the track run, and also to allow for the head of the M5 bolt that holds in the servo bar. This required lots and lots of offering up, taking off a bit more, and offering up again ........

In order to reduce the 'wobble effect', a recess was drilled into the bottom centre of the inner drum to take the pivot bolt:

So the end result, seen from the rear, is this:

The tolerances need to be very fine - there's only a limited amount of room below the drum that doesn't interfere with the top plate of the track run. The M5 pivot bolt head needed to be cut down to fit, which is when we discovered that the hex isn't the same size all the way down ...... But the eventual outcome allows the drum to spin freely, with very little friction from the top flange and no discernable wobble, so the servo should be able to cope without strain (I'll test it with my Turnigy servo adjuster soon - it'll be a long time yet before I fit the electronics).

Another source of friction was the top of the drum, which needs to be clear of the top plate. Mine had a lot of machine marks on it:

which were removed with milling, as well as reducing its height a bit so it now sits within the ring rather than proud of it:

The edge of the top plate was also smoothed a bit to fit properly into the top ring.

And that's it ! A lot more complex than Jerry's solution, but I'm happy with it, and grateful to Stephen for his efforts and expertise. I better crack on and finish my Burma Lee now, hadn't I ?

Best wishes,

Chris

PS. This may not be the end of the saga ! Adrian Harris joined us, and he said he had another way of doing it that needed thinking through. And he's got three M3's to fettle ..........

PPS. To me, the M3 is more of a throwback to WW1. Therefore the outer casing is a sponson, and the inner mount is a drum. Even the build instructions call it a sponson. But, like Jerry says, potato and potato .......

[John Clarke]

Hi Jerry

Liked the clip. hope to do something like it when something other than me moves on the Chieftain.

Have you thought of limiting the speed of the servo controlled Barbette. In the past I have utilized the Fly boys under carriage servo speed controllers on large guns on model ships.

Other than delay starts these things can slow the function of the servo down as if the gun is being layed on target. Removing the need to try and slow the control of the gun position with the transmitter stick, in other words the servo can be regulated and moves in a controlled slomo action to the point you want to go.

I guess some Transmitters can be programmed to do the same job. but this device fits between servo and receiver once setup, it does the job.

The ones I used were made by Turnigy, Servo Speed changer Direction changer Regulator - Turnigy - 3 Channel

s-l500as.jpg (38.22 KiB) Viewed 3396 times

Another on an ebay search, these turned up "Servo Signal Converter Delay Slow Converter RC Model DIY Upgrade Accessories Kit"

Please ignore if you had other plans.

[Stephen White]

Nice write up Chris, glad to help. That’ll be the barbette within the sponson then……